The present invention relates to mass spectroscopy systems, and more particularly, but without limitation, relates to an ion mobility spectrometer system that transmits ions through a drift tube to a collector with a high degree of efficiency.
Ion mobility spectrometers (IMS) differentiate between component molecules in a sample on the basis of the time taken by the component molecules, when ionized, to move through the length of a drift tube containing a drift gas under the influence of an electric field. A typical example of such an ion mobility spectrometer, shown in cross-section in FIG. 1, illustrates this mechanism. Molecules in a sample are ionized in a gaseous phase at an ion source 5, which may be, for example, a radioactivexcex2-emitter, a photoionizer, or a corona-discharge arrangement. The ions generated at the source are then directed into the opening 11 at a first end of the drift tube 10. Within the drift tube 10, a series of ring-shaped electrodes (e.g., electrodes 15, 16) are arranged coaxially and are equally spaced throughout the length of the tube. The ring electrodes 15, 16 are electrically isolated and connected via resistors (e.g., 35, 36) to a voltage divider 40. A d.c. power supply 45 is coupled to the voltage divider 40, whereby the ring electrodes 15, 16 establish a linear potential gradient, typically in the range of 200 V/cm, along the central axis of the drift tube 10 which causes ions within the drift tube to move through the tube in a generally axial direction.
The drift tube 10 is filled with a drift gas, such as helium or nitrogen, at approximately atmospheric temperature and pressure, supplied through a drift gas inlet 21 at the opposite end of the drift tube. Collisions between the ions and drift gas molecules cause the ions to lose kinetic energy. In a steady state, the loss in kinetic energy is balanced by energy gained from the electric field. An equilibrium condition results in which the ions travel in the drift tube at a constant velocity given by:
v=KExe2x80x83xe2x80x83(1)
where K is referred to as the ion mobility constant and E is the axial electric field applied in the drift tube. According to current theoretical models (see Eiceman et al., Ion Mobility Spectroscopy, CRC Press 1993 pgs. 57-86), the mobility constant of a particular ion (at constant temperature and pressure) is related to the reduced mass of the ion and to a diffusion collision integral, which is related to the size and shape of the ion.
During operation, an ion gate electrode 50, usually consisting of a grid mesh mounted perpendicularly to the axis toward the first end of the drift tube 10, acts as a gatekeeper allowing packets of ions, typically several microseconds to several milliseconds in duration, to pass through when an electric pulse is applied to the gate. A collector electrode 60 is placed at the opposite end of the drift tube 10. As ion packets released from the gate 50 drift toward the collector 60, the ions within the packet are separated according to their various ion mobility constants. By measuring the travel times of the ions between the gate and the collector, their respective ion mobility constants can be readily calculated, and their specific masses and shapes can be deduced therefrom.
One of the factors that reduces optimal performance of such conventional IMS instruments is their generally low transmission efficiency due to radial migration of the ions in the drift chamber. This radial migration, mainly caused by initial divergence of the ion beam due to temperature gradients, transverse diffusion of the ions, and space charge repulsion between the ions, results in a large portion of the ions leaving the axial trajectory toward the ring electrodes 15, 16 where they become neutralized on contact. Such losses increase in proportion to the length of the drift tube as the ions have more time to become diverted from an axial trajectory. Thus, even though the resolution capabilities of IMS instruments provide for detecting sample concentrations as low several parts per trillion, their performance suffers because of the low transmission efficiencies, which can be as low as 1%.
What is therefore needed is a means of increasing the ion transmission efficiencies of IMS instruments by reducing radial ion diffusion in the drift tube, without sacrificing the high resolution and other advantageous qualities of these instruments.
The present invention provides an ion mobility spectrometer comprising a drift tube having a central axis along which a plurality of ring electrodes is arranged. The plurality of ring electrodes provide a linear potential gradient along the central axis of the drift tube. In addition, an RF voltage source, coupled to the plurality of ring electrodes, generates an oscillating RF potential within the internal region of each of the ring electrodes. The oscillating RF potential influences radial motion of the ions within the drift tube so that the ions remain within a radial space defined by the internal regions of the electrodes.
In one embodiment of the ion mobility spectrometer according to the present invention, each of the plurality of ring electrodes includes an even number of segments, wherein each segment is electrically insulated, and is maintained at an oscillating RF potential of an opposite polarity from adjacent segments. A multipole electric field is thereby generated within the respective internal region of each of the ring electrodes.
According to another embodiment, the ion mobility spectrometer according to the present invention includes a plurality of disk electrodes. Each of the plurality of disk electrodes is centered in the internal region of one of the plurality of ring electrodes, with each disk being coaxial with the drift tube. The RF voltage source is coupled to each of the plurality of disk electrodes such that an oscillating RF potential is generated between each of the plurality of ring electrodes and the respective disk electrode centered in the ring electrode.
The present invention also provides a method for efficiently transmitting ions along a linear trajectory in an ion mobility spectrometer. Ions are introduced into a cylindrical drift tube having a central axis. A plurality of ring electrodes are arranged in a stack along the central axis of the drift tube, each of the ring electrodes having an internal region perpendicular to the central axis of the drift tube. A linear potential gradient is generated along a central axis of the drift tube to influence motion of ions along the central axis, and an oscillating RF potential is generated within the internal region of each of the axially stacked ring electrodes that constrains radial motion of the ions perpendicular to the axis of stacked ring electrodes by influence of the oscillating RF potential.